This invention relates to the use of copper catalysts in the oxidative coupling of 2,6-dialkylphenols. More specifically, this invention relates to copper amine salt complexes having high catalytic activity in the coupling of 2,6-dialkylphenols to produce mixtures of 3,3', 5,5'-tetra-alkyl-4,4'-dihydroxybiphenyls (also known as 3,3', 5,5'-tetra-alkylbiphenols) and corresponding diphenoquinones.
The prior art discloses that 3,3', 5,5'-tetra-alkylbiphenols, which are important precursors in the synthesis of 4,4'-biphenol, were prepared either from 3,3', 5,5'-tetra-alkyl-4,4'-diphenoquinones or by direct conversion of 2,6-dialkylphenols. The oxidative coupling reaction which produces diphenoquinone structures has, in most instances, been accomplished in the presence of an organometallic complex which acts as a catalyst. The use of a catalyst allows the reaction to be carried out at lower temperatures under milder conditions.
The concentration of the catalyst is an important consideration in this process. In general, the use of high catalyst concentrations results in shorter reaction times, but this is counter balanced by high levels of catalyst residuals in the product, which require elaborate purification measures for removal. Therefore, the oxidative coupling of 2,6-dialkylphenols with a minimum quantity of catalyst is highly desirable to maintain reaction times of reasonable duration.
Formation of tetra-alkylbiphenols via diphenoquinones can be accomplished either by (a) standard quinone reduction techniques, which reduces product yield, or (b) through a disproportionation reaction in which the diphenoquinone reacts with two molar equivalents of the appropriate 2,6-dialkylphenol to give two molar equivalents of the tetra-alkylbiphenol. The disproportionation reaction is further described in U.S. Pat. No. 3,631,208.
An improvement on this technology involves eliminating the disproportionation step by directly converting 2,6-dialkylphenols to 3,3', 5,5'-tetra-alkylbiphenols during the oxidation step as illustrated below: ##STR1##
One stage processes for this oxidation are disclosed in the following U.S. patents, but all of these methods necessitate stringent reaction conditions, difficult product work-up procedures and high concentrations of catalyst.
U.S. Pat. No. 3,812,193 discloses the oxidation of 2,6-diisopropylphenol to the corresponding biphenol using a ferric aqueous medium. These processes incorporate product isolation procedures which are exceedingly difficult to scale-up to a commercial level.
A one-step procedure to alkylated biphenols is described in U.S. Pat. No. 4,180,686. Palladium acetate was found to catalyze the oxidation of di- and trialkylated phenols to corresponding alkylated biphenols in polar organic solvents. The major disadvantages with this process are the requirements for high oxygen pressure (50-100 psig) and the positions of phenol ring functionality. In order to avoid large amounts of diphenoquinone formation, at least one alkyl group must be meta to the phenol function. This process is also complicated by the use of organic solvents which must be removed in work-up.
U.S. Pat. No. 3,247,262 demonstrates the oxidation of 2,6-dialkylphenols to the tetra-alkylated biphenol without a solvent. The reaction was performed in the presence of stoichiometric quantities of a cupric carboxylate salt at rather high reaction temperatures of 140.degree.-255.degree. C.
U.S. Pat. No. 4,195,189 discloses a similar type of oxidation which utilizes an activated basic cupric oxide having a surface area of approximately 5-50 square meters/gram. This process has the disadvantage of requiring a 1.0 to 1.8 ratio of cupric oxide to 2,6-dialkylphenol. The product, therefore, must be leached away from the cupric oxide with a polar organic solvent or a halogenated hydrocarbon, thus complicating the process.